Field of the Invention
The invention relates to an imaging device and a method for forming an image on a printing form for an offset printing press. In this regard, a special material is used for the printing form.
The filmless production of printing forms for offset printing is increasingly expanding. For digital printing form production, an imaging unit is used that receives image data from a computer and converts this data directly into a material image on the surface of the printing form. A conventional technique for printing form imaging uses a laser that vaporizes an ink-repellent covering layer on the printing form in image regions, so that an ink-accepting layer located therebeneath accepts the printing ink.
Laser imaging units are very complicated, contain components with a limited service life and frequently require maintenance. In addition, the vaporization of the covering layer produces residues, which must be removed carefully.
It is accordingly an object of the invention to provide an imaging device and a method of forming an image of a printing form for an offset printing press which are relatively simple, utilize components which have a durable service life, require minimal maintenance, and avoid residue formation.
With the foregoing and other objects in view, there is provided, in accordance with one aspect of the invention, an imaging device for forming an image on a printing form, comprising an imaging head containing a multiplicity of mutually parallel conductive pins separated from one another by an insulating material, a voltage source for a high-frequency alternating voltage, and a multiplicity of electronic switches, respectively, connected at one end thereof to one of the conductive pins, and at the other end thereof to the voltage source.
In accordance with another feature of the invention, the conductive pins are embedded in a regular arrangement in the insulating material, the insulating material having an outer surface whereon, respectively, an end of each of the conductive pins is exposed.
In accordance with another aspect of the invention, there is provided a method of forming an image on a printing form, which comprises exposing a printing form surface formed of a polar material having properties selected from the group thereof consisting of hydrophilic and hydrophobic properties to high-frequency alternating electric fields, in regions selected from the group thereof consisting of image regions and non-image regions, the intensity, the frequency and the duration of action of the high-frequency alternating electric fields being selected so that the respectively selected hydrophilic and hydrophobic properties of the polar material of the printing form are changed in the respectively selected image regions and non-image regions.
In accordance with an additional mode of the method of the invention, the polar material is a polymer with hydrophobic properties which become hydrophilic in non-image regions due to the action of the high-frequency alternating fields.
In accordance with a concomitant aspect of the invention, there is provided a printing form for an offset printing machine having an outer surface, comprising a material forming the surface, the material having properties selected from the group thereof consisting of hydrophilic and hydrophobic properties, the material being a polar material.
The problems of the prior art are thus solved by an imaging device having an imaging or image-setting head containing a multiplicity of mutually parallel conductive pins which are separated from one another by an insulating material, a voltage source for a high-frequency alternating voltage, and a multiplicity of electronic switches, each of which is connected at one end thereof to one of the conductive pins, and at the other end thereof to the voltage source. The conductive pins are preferably embedded, in a regular arrangement, in the insulating material, which has an outer surface on which an end of each pin is exposed.
A printing form that is suitable for imaging or image setting using such an imaging head has a surface containing or consisting of a polar material having either hydrophilic or hydrophobic properties. A polar material is a material which contains polar molecules, i.e., molecules having a dipole moment even in a field-free space. In an electric field, the molecules of the polar material align themselves preferentially in the field direction. This process requires a given amount of time, which is all the longer the more viscous the surrounding medium is. In high-frequency alternating fields, therefore, the dipole setting lags behind the field. In this regard, electrical field energy is converted into heat energy, which can destroy the polar material due to the breaking apart of the bonds between the molecules. This effect is utilized, for example, for welding polar polymers such as PVC by high frequency alternating voltages.
The invention makes use of the circumstance that the decomposition of a polar material is normally also accompanied by a change in the surface properties thereof. For example, the originally hydrophobic properties of a polymer are changed to hydrophilic when it is decomposed thermally, because the surface thereof becomes rough and porous.
In the method of the invention for forming or setting an image on a printing form having a surface with the aforementioned properties, the printing form surface is exposed to high-frequency alternating electric fields either in image regions or in non-image regions thereof, the intensity, the frequency and the duration of action of the high-frequency alternating electric fields being selected so that the hydrophilic or hydrophobic properties of the polar material of the printing form are changed in the image regions or non-image regions, i.e., the properties are neutralized or even reversed with respect to the counterpart thereof. In this regard, the dot or point-like regions on the printing form surface which are converted by the alternating electric field also become more-or-less oleophobic or oleophilic, as needed for the offset printing process.
In order to perform this method with the aid of the imaging or image-setting head according to the invention, the latter is brought close to the printing form surface, and the electronic switches are switched on or off selectively, regions on the printing form which are located opposite conductive pins to which the high-frequency alternating voltage is applied being exposed to the high-frequency alternating electric field and consequently converted.
If the polar material of the printing form is a polymer that is normally hydrophobic, the non-image regions are exposed to the high-frequency alternating electric field, the surface properties in the non-image regions being changed to hydrophilic. In the case of a normally hydrophilic printing form material, the image regions would have to be converted accordingly. During a printing operation, the printing form is dampened in a conventional manner and is subsequently inked, only the image regions accepting the printing ink.
The imaging head according to the invention has a construction that is very much simpler and more rugged than a conventional laser imaging unit. Although a large number of electronic switches are needed, these are easy to mass-produce. For example, suitable switching transistors and the necessary connecting lines can be formed directly on the rear of the pin group with the aid of techniques such as have become known heretofore from semiconductor production.
Otherwise than in laser imaging, in the case of the invention, virtually no residues are produced in the form of loose particles which can impair the printing process.
The imaging head is preferably arranged opposite the printing form, either in the printing machine itself (xe2x80x9con-pressxe2x80x9d) or outside the printing machine in a separate imaging unit (xe2x80x9coff-pressxe2x80x9d). For xe2x80x9con-pressxe2x80x9d imaging, the untreated printing form or printing plate, formed of a carrier layer and a polar material applied thereto, is clamped onto a plate cylinder, and the image is formed or set by an imaging head installed in the printing machine, in order to produce a printing form that can be used for offset printing. Alternatively, a film of polar material adhesively attached to the plate cylinder, for example, can be used. For xe2x80x9coff-pressxe2x80x9d imaging, the printing plate or film is formed or set with an image outside the printing machine by an imaging head and is then applied to the plate cylinder. During the imaging process, the distance between the printing form and the imaging head should be as small as possible, without contact therebetween. However, it is assumed that if the printing form and the imaging head make mutual contact, no damage occurs.
Alternatively, the imaging head can be integrated into the cover of a plate cylinder, the exposed ends of the pins pointing outwardly. In this case, a film of polar material that is clamped or adhesively attached to the plate cylinder is used as the printing form. It may be expedient, in this case, to use an external counterelectrode, for example a grounded metal plate, so that unconverted regions in the film which correspond to image points or non-image points reliably extend as far as the outer side of the film.
The spacing or distances between the conductive pins, and the diameter thereof, which should be made as small as technically possible, are critical for the resolution of the imaging head. In an embodiment in which only the alternating electric fields are responsible for the conversion of the printing form material, the distances between the conductive pins are limited by the fact that, with the given voltage, flashovers between the points of the pins and the printing plate do not, in fact, occur. In this way, low and medium resolutions are achievable by the invention. Higher resolutions can possibly be achieved by a further feature or development of the invention in which flashovers between the pins and the printing form are not only tolerated but are used to assist the conversion of the printing form material. The high-frequency alternating voltage used in the invention offers the advantage that it does not cause any electrode burnup, as opposed to direct voltage or low-frequency alternating voltage.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an imaging device and a method for forming an image on a printing form for an offset printing machine, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, wherein: